Transmitters form one half of most communication circuits. As such, they assume a position of prominence in design concerns. With the proliferation of mobile terminals, transmitter design has progressed in leaps and bounds as designers try to minimize components and reduce size, battery consumption, and the like. Likewise, modulation schemes are continuously updated to reflect new approaches to maximize information transfers in limited bandwidths. Changes in standards or standards based on newly available spectrum may also cause designers to approach modulating transmitters with different techniques.
Many different standards and modulation schemes exist, but one of the most prevalently used in the world of mobile terminals is the Global System for Mobile Communications (GSM). GSM has many variants, not the least of which is General Packet Radio Service (GPRS). GPRS is a new non-voice value-added service that allows information to be sent and received across a mobile telephone network. It supplements today's Circuit Switched Data and Short Message Service. GSM allows many different types of mobile terminals, such as cellular phones, pagers, wireless modem adapted laptops, and the like, to communicate wirelessly through the Public Land Mobile Network (PLMN) to the Public Switched Telephone Network (PSTN).
One relatively recent change has been the advent of the Enhanced Data for GSM Evolution (EDGE) scheme in GSM systems. This system contains amplitude modulation components, and, as a result, the power amplifier must be linear, never operating in saturation if classical modulation techniques are employed. Such a system lacks the efficiency of one that operates the power amplifier in saturation.
If a polar modulation system is used instead of a classical modulation system, then the power amplifier may operate in saturation and efficiency would be greatly improved. In addition, if the polar signals are generated by a digital method, such a system does not require the use of a high current drain quadrature modulator. Quadrature modulators are undesirable from a design standpoint in that they draw large amounts of current, and hence, drain batteries comparatively fast.
Unfortunately, the amplitude signal that controls the power amplifier will cause unwanted phase components to be created in the output of the power amplifier due to the non-linearities of the power amplifier. This is sometimes called Amplitude Modulation to Phase Modulation (AM/PM) conversion, or AM/PM distortion, and it degrades the Output Radio Frequency Spectrum (ORFS) of the system and the Error Vector Magnitude (EVM). Thus, a need also exists to be able to counteract or eliminate the unwanted AM/PM distortion signal from the transmitted phase signal.
An additional concern is that the power amplifier may have a non-linear gain with varying output power. This may create what is called Amplitude Modulation to Amplitude Modulation (AM/AM) conversion, which is also referred to as AM/AM distortion. The AM/AM distortion may have both phase and amplitude distortion components, and to create a better control system, these should be reduced or eliminated as well.
One method of reducing AM/PM and AM/AM distortion is to provide predistortion of the amplitude and phase path in order to compensate, or substantially cancel, the AM/PM and AM/AM distortion. However, optimization of the predistortion is difficult because the AM/PM and AM/AM distortion can not be easily measured or calculated. Further, AM/AM error, AM/PM error, and misalignment in time between the amplitude and phase paths will appear as degraded EVM and ORFS. Optimization is further complicated because a change in predistortion may improve any one spectrum offset of the ORFS while it degrades other spectrum offsets as well as EVM.
Thus, there remains a need for a system and method for optimizing AM/AM and AM/PM predistortion such that EVM and desired offset frequencies within the ORFS are minimized.